Avoid database lock errors by always updating progress in the main thread

[kollivier]

Summary

It's possible to encounter SQLite database locked errors when concurrently writing progress updates from multiple threads. This fix prevents this from happening by having the main import thread perform all db updates.

Reviewer guidance

Testing import when running Kolibri on slower devices like Raspberry Pi and Android with larger channel imports should be enough to confirm that this is fixed.

References

Fixes #7673

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[rtibbles]

Code changes make sense - @radinamatic would you be able to take an asset for a spin to test?

[jredrejo]

It works fine, and I can confirm it solves #7673 in a RPi, thanks!